Image compression systems are well known. The invention will be described with particular reference to the JPEG system, but may also be used with other systems. JPEG compression is commonly used in applications such as cameras. In cameras incorporated in mobile phones, there are severe memory limitations and the camera system is set up to impose a limit on the size of a compressed image. The size of the limit is chosen based on the specifications of the phone and its camera system, but a typical value is 12 kb. It will be appreciated that this value could be lower or higher based on the specification of the phone. The value could be different by an order of magnitude when referring to other devices.
Such limits can be imposed by forming a compressed image at a first compression factor, comparing the byte size of the compressed image against the size limit, and if the size limit is exceeded, then forming a second compressed limit at a higher compression factor.
Many scenes will give an acceptable image within such a limit, but where a scene contains a significant amount of fine detail the resulting compressed image will frequently be of an unacceptable quality, with the image being formed of excessively large pixels. A similar problem can exist as a result of low light levels rather than excessive detail.